As worldwide petroleum deposits decrease, there is rising concern over shortages and the costs that are associated with the production of hydrocarbon products. As a result, alternatives to products that are currently processed from petroleum are being investigated. In this effort, biofuel such as biodiesel has been identified as a possible alternative to petroleum-based transportation fuels. In general, biodiesel is a biofuel comprised of mono-alkyl esters of long chain fatty acids derived from plant oils or animal fats. In industrial practice, biodiesel is created when plant oils or animal fats are reacted with an alcohol, such as methanol.
For plant-derived biofuel, solar energy is first transformed into chemical energy through photosynthesis. The chemical energy is then refined into a usable fuel. Currently, the process involved in creating biofuel from plant oils is expensive relative to the process of extracting and refining petroleum. It is possible, however, that the cost of processing a plant-derived biofuel could be reduced by maximizing the rate of growth of the plant source. Because algae is known to be one of the most efficient plants for converting solar energy into cell growth, it is of particular interest as a biofuel source. Importantly, the use of algae as a biofuel source presents no exceptional problems, i.e., biofuel can be processed from oil in algae as easily as from oils in land-based plants.
While algae transforms solar energy into chemical energy more efficiently than other plants, large-scale algae harvest operations do not optimize photoefficiency for algae in a cost effective manner. For instance, enclosed systems are expensive due to the energy costs invested in providing light energy to the algae. On the other hand, outdoor raceway systems offer poor access to light energy for most of their algae. Specifically, algae near the surface grow densely and block the solar energy from reaching algae deeper in the stream. Further, algae cells take 0.5 to 2 seconds to process photons of light. During this processing time, light energy that is absorbed by the algae cell is not converted to chemical energy. Dynamic mixing operations further complicate and increase costs.
In light of the above, it is an object of the present invention to provide a system and method for optimizing photoefficiency for algae cells grown in a bioreactor. For this purpose, a number of systems have been developed, such as those disclosed in co-pending U.S. Patent application Ser. No. 11/549,532 for an invention entitled “Photosynthetic Oil Production in a Two-Stage Reactor,” co-pending U.S. Patent application Ser. No. 11/549,541 for an invention entitled “Photosynthetic Carbon Dioxide Sequestration and Pollution Abatement” and co-pending U.S. Patent application Ser. No. 11/549,561 for an invention entitled “Photosynthetic Oil Production with High Carbon Dioxide Utilization,” which are filed concurrently herewith and assigned to the same assignee as the present invention, and are hereby incorporated by reference. Another object of the present invention is to provide a system for growing algae in a fluid stream which causes algae cells to surface for predetermined intervals of time. Still another object of the present invention is to provide a system for growing algae cells moving in a fluid stream, and to utilize simple barriers to control the flow of the fluid stream. Another object of the present invention is to provide a bioreactor system for growing algae in a fluid stream that defines a flow path for sequential movement of the algae during cell growth to and from the surface of the fluid stream. Still another object of the present invention is to create von Karman vortices in a fluid stream of algae cells to sequentially flow algae cells to the surface to receive light energy for a predetermined interval of time. Yet another object of the present invention is to provide a system and method for producing algae with high photoefficiency that is simple to implement, easy to use, and comparatively cost effective.